The invention relates to a process for the preparation of (S,S)-N,Nxe2x80x2-ethylenediaminedisuccinic acid ((S,S)-EDDS), analogous compounds or their salts. (S,S)-EDDS has the following formula: 
(S,S)-EDDS is used as a biodegradable complexing agent for alkaline earth metal ions and heavy metal ions. Various processes for their preparation are known.
For example, (S,S)-EDDS can be prepared using microorganisms. Thus, alkylenediamines and fumaric acid can be reacted in the presence of Burkholderia sp., as is described in EP-A-0 731 171.
The preparation can also be carried out by reaction of L-aspartic acid with glyoxal in the presence of bases, and subsequent reduction with sodium borohydride. A process of this type is described, for example, in WO 96/32371.
The preparation can also be carried out by reaction of L-aspartic acid with glyoxal in the presence of bases, and subsequent reduction with sodium borohydride. A process of this type is described, for example, in WO 96/32371.
GB-A-2 299 809 describes the reaction of methyl L-aspartate with 1,2-dibromoethane.
According to WO 96/01801, the preparation of (S,S)-EDDS can be prepared by reaction of L-aspartic acid with 1,2-dibromoethane in water.
According to WO 94/28464, (rac)-EDDS is prepared by reaction of ethylenediamine and maleic acid.
The microbial processes for the preparation of (S,S)-EDDS are technically complex and produce the desired product only in a low yield. The other known processes likewise produce the desired product only in a low yield. The use of expensive chemicals is often necessary. The use of halogen-containing organic compounds always results in a certain content of organically bonded halogen (AOX). For environmental protection reasons, the formation of AOX should be avoided as far as possible.
It is an object of the present invention to provide a process for the preparation of (S,S)-EDDS and analogous compounds, which produces (S,S)-EDDS in a high yield starting from cost-effective, readily available starting materials in a straightforward process.
We have found that this object is achieved according to the invention by a process for the preparation of (S,S)-N,Nxe2x80x2-ethylenediaminedisuccinic acid or analogous compounds or salts thereof by reaction of L-aspartic acid, L-glutamic acid or L-cysteic acid with 1,2-bissulfooxyethane, 1,3,2-dioxathiolane 2,2-dioxide, mixtures thereof or 1,3-bissulfooxy-2-hydroxypropane in the presence of a base. and optional subsequent acidification with an acid.
The object is further achieved according to the invention by a process for the preparation of (S,S)-N,Nxe2x80x2-ethylenediaminedisuccinic acid or analogous compounds or salts thereof by reaction of N-(2-sulfooxyethyl)-L-aspartic acid or its salts with L-aspartic acid or corresponding compounds based on L-glutamic acid, L-cysteic acid and/or 1,3-bissulfooxy-2-hydroxypropane in the presence of a base. and optional subsequent acidification with an acid.
The invention further relates to a process for the preparation of N-(2-sulfooxy-ethyl)-L-aspartic acid or analogous compounds or salts thereof by reaction of L-aspartic acid, L-glutamic acid or L-cysteic acid with an equimolar amount of 1,2-bissulfooxyethane, 1,3,2-dioxathiolane 2,2-dioxide or mixtures thereof or 1,3-bissulfooxy-2-hydroxypropane in the presence of a base, and optional subsequent acidification with an acid.
According to the invention, it has been found that the preparation of (S,S)-EDDS by reaction of L-aspartic acid with ethylene glycol disulfuric acid (1,2-bis-sulfooxyethane, CAS: 6914-91-6) can take place in accordance with the following reaction scheme: 
1,2-Bissulfooxyethane can be obtained by reaction of ethylene glycol with sulfur trioxide, chlorosulfonic acid, sulfuric anhydride or sulfuric acid. The synthesis is described, for example, in J. Prakt. Chem. 1879, 20, 2. The reaction of sulfur trioxide with ethylene glycol is described, for example, in J. Am. Chem. Soc. 1954, 76, 5361.
Instead of 1,2-bissulfooxyethane, it is also possible to use the cyclic glycol sulfate (1,3,2-dioxathiolane 2,2-dioxide, CAS: 1072-53-3). Glycol sulfate can be prepared, for example, by the process described in DE-A-20 40 503. The reaction takes place in accordance with the following scheme: 
Instead of 1,2-bissulfooxyethane or 1,3,2-dioxathiolane 2,2-dioxide, it is also possible to use a mixture of both compounds. The reaction is carried out using 2 equivalents of L-aspartic acid. Furthermore, the reaction can also be carried out sequentially. For this, L-aspartic acid is firstly reacted with an equivalent of 1,2-bissulfooxyethane or 1,3,2-dioxathiolane 2,2-dioxide or mixtures thereof to give N-(2-sulfooxyethyl)-L-aspartic acid or its salts. This compound is then reacted with an equivalent of L-aspartic acid in the presence of bases to give (S,S)-EDDS in accordance with the following scheme: 
The bases used in the reactions can be any suitable bases. Examples are alkali metal and alkaline earth metal hydroxides, alkali metal and alkaline earth metal carbonates, tertiary amines or ammonia, and mixtures thereof. Preference is given to using sodium hydroxide, sodium carbonate, potassium hydroxide, potassium carbonate or calcium hydroxide. If ammonia is used, any ammonium sulfate which forms can be reconverted into ammonia and sulfuric acid in a simple manner.
During the reaction, the pH is preferably maintained in the range from 7 to 13, preferably from 7 to 11, particularly preferably from 8 to 10. After the reaction, the product can be acidified with an acid. Suitable acids are, for example, organic carboxylic acids, such as acetic acid, or inorganic acids, such as sulfuric acid. If the product is completely acidified, then free (S,S)-EDDS is obtained. This can be isolated by mechanical separation (filtration) since it precipitates out of the reaction mixture. In this case, all carboxyl groups in the (S,S)-EDDS are in protonated form. By omitting the acidification with an acid or acidification with a smaller amount of acid, salts of (S,S)-EDDS can be obtained in which 1 to 4 of the carboxyl groups are present in salt form. Thus, mono-, di-, tri- or tetra-salts, for example, can be prepared. In the salts, the counterion present is the ion of the corresponding base used. If sodium carbonate or sodium hydroxide are used, then mono-, di-, tri- and tetrasodium salts of (S,S)-EDDS can be obtained. If the corresponding potassium compounds are used, the corresponding potassium salts are obtained. In product mixtures, on average, uneven values for the number of salt groups in the molecule can also be obtained.
If calcium hydroxide is used as the base, the corresponding calcium salts are firstly obtained, which can be converted into the sodium salts by adding sodium carbonate and separating off the sparingly soluble calcium carbonate.
The reaction is preferably carried out in aqueous solution. A preferred method involves suspending L-aspartic acid in water and adding base until the desired pH is established. Then, 1,2-bissulfooxyethane, 1,3,2-dioxathiolane 2,2-dioxide or mixtures thereof are gradually introduced into the reaction mixture. During the reaction, the reaction temperature should preferably not exceed 50xc2x0 C., particularly preferably 30xc2x0 C., in particular 25xc2x0 C. During the addition, the pH is maintained at the desired value by adding base. When the addition is complete, the reaction mixture is preferably heated to a temperature in the range from 30 to 100xc2x0 C., particularly preferably 60 to 100xc2x0 C., in particular under reflux. The progress of the reaction can be monitored, for example, by HPLC (High Performance Liquid Chromatography). As soon as no further decrease in the amount of L-aspartic acid can be detected, the reaction mixture is cooled and worked up. For example, the reaction mixture can be rendered acidic using sulfuric acid, and the precipitated (S,S)-EDDS can be separated off mechanically, for example by filtration or centrifugation. When the reaction is complete, the reaction mixture can also be spray-dried without acidification. In this process a mixture of (S,S)-EDDS tetrasodium salt, L-aspartic acid disodium salt, ethylene glycol and sodium sulfate is obtained if the base used was sodium hydroxide solution. If other bases are used, the corresponding salts are obtained.
Corresponding reactions are possible with L-glutamic acid, L-cysteic acid, 1,2-bissulfooxyethane and 1,3-bissulfooxy-2-hydroxypropane. In each case, S,S-configurations are obtained, e.g. (S,S)-EDDG and (S,S)-HPDDS, i.e. (S,S)-ethylenediaminediglutamic acid and hydropropylene-1,3-diaminodisuccinic acid.